The present invention relates to a scale formation-inhibitory agent, which is used for preventing the formation, during welding, of any scale on the surface of a metal part to be welded during a welding treatment as well as a method for inhibiting formation of a scale in a welding treatment of metal parts.
Conventionally, Japanese Examined Patent Publication No. Sho 61-2753, for instance, discloses such agents for inhibiting, through heating, the discoloration of copper and copper alloys as those comprising boric acid and/or salts thereof. In addition, Japanese Patent No. 2504673 discloses a method which comprises the steps of applying a stock solution of a boric acid ester, a solution containing a boric acid ester or a suspension of boric acid powder to a portion to be joined and then heating the portion, as a method for brazing and soldering brass and copper parts or brass parts. In the technique disclosed in this patent, for instance, boric acid or a salt thereof is heated to thus form a glass-like coating film on the metal surface and the resulting film isolates the metal surface from the air to thus inhibit oxidation of metallic tubings. Therefore, the technique disclosed in the foregoing patent may suffer from such a problem that such a glass-like material is peeled off from the welded metallic tubings and if the welded tubings are used in, for instance, the piping for refrigerators and air conditioners, the glass-like material peeled off is transported to the compressors thereof during operation and this leads to, for instance, breakage of valves and/or seals.
Moreover, there has also been proposed a technique for inhibiting oxidation by applying an organic acid ester to the interior of a tubing when welding the tubing to thus make use of the reducing power of the organic acid ester or the thermal decomposition products thereof formed during heating the tubing at a high temperature (Japanese Un-Examined Patent Publication No. Hei 8-132239), but high molecular weight organic acid esters form soot and/or tar materials and this results in the formation of scale. On the other hand, low molecular weight organic acid esters which do not form soot or the like are quite volatile substances and they must therefore be used in a large quantity. In addition, the use of these substances requires a high airtightness in the interior of the portion to be joined.